1. Field of the invention
This invention relates, in general, to a process for changing the wetting agents of solids, and more particularly, to a process for changing the wetting agent of pigments.
2. State of the Art
Pigments are typically produced in an aqueous phase to diminish handling problems. However, as a final product, the pigments are, generally, desired to be in an oily phase. It is known to dry pigments before contacting them with the desired oily agent. However, this method is relatively expensive.
When removal of a liquid from a solid phase is required, it is common to use compression forces exerted onto the mixture. Examples can be found in U.S. Pat. Nos. 5,417,854; 5,466,370; 5,480,541 and 5,489,381. U.S. Pat. Nos. 5,151,026 and 5,232,649 disclose extracting a liquid from polymers or rubber materials by an extruder. If one would apply such an extruder for drying pigments or other solid material to separate the solid phase, the expenses would again be high, because time and energy has to be invested for subsequently contacting the pigments or other solid material with the oily agent.
Other examples of using compression forces to separate solids from liquids can be found in U.S. Pat. No. 5,160,441; U.S. Pat. No. 5,205,930; U.S. Pat. No. 3,035,306; and U.S. Pat. No. 4,474,473.
U.S. Pat. No. 4,474,473 also discloses use of compressive forces, and teaches using a screw having relatively shallow channels, and including special screws having deep channels only in a middle part of the barrel. The use of shallow channels, with practically no free space within these channels, limits the effectiveness with which water is removed via a vent 15. That is, in the vicinity of vent 15, there is only limited opportunity to remove water which may have been trapped in the material.
It is an object of the present invention to provide a more time and cost effective process of separating liquid and solid phases.
It is a further object to change the wetting agent in a three phase system, where a solid phase is dispersed in a first aqueous liquid phase which is exchanged with a second, oily phase.
These and other objects are achieved in a first inventive step with the recognition that oily matters have normally a higher adherence to solid matters than water.
In a second step, a conventional extruder can be used in a conventional manner to separate the solid phase from the first aqueous liquid phase, where the extruder applies three compression forces on a pinch point of the material. However, it was found that compressing material from three sides does not optimally effect liquid extraction when a three-phase system is used, i.e. a first phase of solid pigments, a second aqueous phase and a third, oily phase. In an investigation of this phenomenon, it was discovered that compressing material from three sides entraps water within the mass, not allowing it to escape. Although additional water extraction stages can be provided for serially extracting water from the material, exemplary embodiments of the present invention are directed to avoiding the time and expense of these additional stages.
Known extruders exert one of the three aforementional forces using an auxiliary extruder extending laterally and perpendicularly to the main extruder. This auxiliary extruder rotates in a barrel which has a feeding hopper. Thus, part of the material to be dried is introduced through this auxiliary extruder. Under these conditions, one of two problems will result: either the auxiliary extruder is of a different length then the main extruder, such that material to be introduced through the auxiliary extruder is treated with a different intensity (that is, different time and/or pressure) than the material which is introduced via the main extruder; or, if the material is to be treated over the same period of time as the material in the main extruder, the auxiliary extruder is about the same length as the main extruder from the supply hopper up to the point where the auxiliary extruder feeds material and exerts one of the three aforementioned forces. The first configuration, with a shorter auxiliary extruder, results in an unequal treatment of the material and thus, an inhomogeneous product of undesirable quality. The second configuration renders the system extremely expensive, cumbersome and space wasting. If, according to such a conventional extruder, further auxiliary extruders have to be arranged downstream, their length must increase accordingly.
Further investigations led to the surprising discovery that all difficulties can be avoided if the rate of introduction of the materials and the speed of the extruder screw (or screws) are selected in such a manner that only part of the space between the adjacent screw threads receive solutions, thus leaving a free space for the aqueous phase to escape. Moreover, it has been found that, although the invention can be used where the solid phase is formed by pigments, particularly for inks, it can be utilized to change the respective liquid phase regardless of the solid phase. Likewise, the liquid phases can be of any type, provided one of at least two such phases has a greater affinity to the solids than the other.
The foregoing is supported by the kneading and shearing effect of the extruder screw within its barrel. This effect on the one hand, provides an intimate contact between the solids, particularly the pigments, and the oily phase, thus releasing water from their surfaces, while the kneading and shearing action ensures that any entrapped water is moved to the free outer surface of the mass where it escapes through at least one lateral opening of the barrel which will not be jammed by compressed material, as is the case with conventional extruders.